1. Field of the Invention
The present invention is related to an image processing method and related apparatus for an image processing system, and more particularly, to an image processing method and related apparatus capable of effectively suppressing overshooting and background noise.
2. Description of the Prior Art
In a prior art image processing system, an image-processing unit may perform image operations for digital image signals, such as scanning-type transition, scaling, edge detection and compensation, color space transformation, etc. Some of the operations need low-pass filtering operations, which may decrease image sharpness. For example, an NTSC television broadcast display system, said NTSC system hereinafter, displays images by interlacing scanning. The interlacing scanning is initially utilized for CRT devices, which scans a display panel twice for each picture by dividing each picture into two fields. One field is composed of odd scan lines, and the other field is composed of even scan lines. To change from interlacing to line-by-line scanning, a low-pass filter is used, so that image sharpness is decreased. In addition, an image scaling operation may also reduce image sharpness. The image scaling operation magnifies or reduces image size to conform a specified format. For example, a 4:3 NTSC image must undergo horizontal and vertical scaling procedures to be displayed on a 16:9 display device. That is, signals received by a TV must be performed the scaling operation to conform to the specified display format, and thus low-pass and interleaving operations are needed. As a result, image sharpness is decreased.
In general, image-processing procedures having the low-pass and interleaving operations inevitably lose image details. Therefore, the prior art usually adds sharpness control steps during the image processing procedures. A common way is to use a peaking process for sharpening image edges, so as to enhance the quality of the image edges. However, the prior art peaking processing may cause overshooting of image edges and decrease image quality accordingly.
Please refer to FIG. 1. FIG. 1 is a block diagram of a prior art sharpness control unit 10. The sharpness control unit 10 includes a high-pass filter 100, a coring operation unit 102, a multiplier 104, and an adder 106. The high-pass filter 100 performs high-pass filtering operations for input image signals. Then, the coring operation unit 102 performs nonlinear coring operations for the filtered image signals with an input-to-output relation as shown in FIG. 2. As shown in FIG. 2, data adjacent a noise level (represented by the threshold Th) can be used for eliminating noise and decreasing values below the threshold Th to zero. The multiplier 104 multiplies the operation results of the coring operation unit 102 by an SF (Scaling Factor, or named a gain) for adjusting and combines signal values with the original image.
In the sharpness-control unit 10, high-frequency data pass through the high-pass filter 100, so that the sharpness procedure enhances high-frequency image edges, meaning that only dots and diagonal lines can be enhanced. In other words, using the sharpness-control unit 10, the sharpness of Hard Edges, obvious and clear edges with high contrast or severe brightness variation, can be dramatically increased, while the sharpness of Soft Edges, unobvious and blur edges with tiny brightness variation, is only slightly enhanced. The sharpness-control unit 10 uses the multiplier 104 to adjust the gain of the image signals. A ratio of Soft edges to the high-pass-filtered image signals provided by the high-pass filter 100 is small, so that even if the gain is increased, the sharpness of Soft Edges can be slightly enhanced. Oppositely, a ratio of Hard Edges to the high-pass-filtered image signals provided by the high-pass filter 100 is high, so that even if the gain is not high, the sharpness of Hard Edges can be effectively enhanced. However, such situation is not the ideal case, because Hard Edges are originally defined as clear edges in the images, and Soft Edges are supposed to be enhanced. Nevertheless, if the scaling factor SF is increased to a value capable of enhancing Soft Edges, Hard Edges will be overshot and background noise will be amplified. Therefore, to prevent overshooting of Hard Edges, the prior art sharpness control unit 10 provides limited sharpness enhancement for Soft Edges.